'screen voltage too high SE 50L6 amp'
Author:Mors08 (registered user: 92 posts )
Date: Tue, Oct 26th, 2010 @ 01:03 ( . )

been building this amp for a while - it's finally working
[link]

the amp sounds pretty cool but my voltages are still off.

my problem lies in the 50L6 output tube.
Pin 4 is still higher than Pin 3.

I have placed a 470 ohm resistor in between the B++ node and pin 4 and no real change. I've even as much as triple the 1.5k resistor in the power section and no change.

Pin 4 is around 122 of the called for 115 while pin 3 is lower at 108 of 120. I've also measured my B+, B++, and B+++ points from where they are noted on the schematic and both B+ and B+++ are fine but the B++ is high at 122 of 115 called for volts.

Why can't I get pin 4 down/ why is pin 4 grabbing so much neg current? Sub question - is this harming the amp or its tone in any way? Is my bias too hot then with too much neg current meaning I'm going to burn through tubes faster? Is there anything actually wrong with the current setup?

This may be stupid but is there any difference at all between a 50L6 and 50L6GT???

-- REPLY: [With No Quote] --- [With Quoted Text]

'screen voltage too high SE 50L6 amp'
Author:trobbins (registered user: 1010 posts )
Date: Tue, Oct 26th, 2010 @ 07:41 ( . )

Have you measured the dc resistance of the OT primary, and can you measure the anode, screen and cathode DC currents at idle. Eg. put a sense resistor in to the B+ line to measure anode current, and perhaps put a 1k in the screen takeoff from B++ to measure DC voltage across it.

Tim

-- REPLY: [With No Quote] --- [With Quoted Text]

'screen voltage too high SE 50L6 amp'
Author:Doug H (registered user: 5398 posts )
Date: Tue, Oct 26th, 2010 @ 08:08 ( . )

On 10/26/2010 @ 01:03, Mors08 wrote :
Is there anything actually wrong with the current setup?

--



What is the cathode voltage of the 50L6?

-- REPLY: [With No Quote] --- [With Quoted Text]

'screen voltage too high SE 50L6 amp'
Author:Mors08 (registered user: 92 posts )
Date: Tue, Oct 26th, 2010 @ 14:08 ( . )

On 10/26/2010 @ 08:08, Doug H wrote :
On 10/26/2010 @ 01:03, Mors08 wrote :
: Is there anything actually wrong with the current setup?
:
: --
:
:

:
: What is the cathode voltage of the 50L6?
--


Cathode voltage is about 10v

I notice this website [link] (at the bottom) says the red lead should go to B+ power.
My book told me to put the blue lead there and the red on B+++
Could swapping these help?

I can't change this until tonight but could this be a problem?
That same site says that a 50L6 output tube should have an OT with a 2K ohm primary. My book called for an 8K - problem?
How do I check the primary resistance of my current OT?

-- REPLY: [With No Quote] --- [With Quoted Text]

'screen voltage too high SE 50L6 amp'
Author:steve25 (registered user: 2153 posts )
Date: Tue, Oct 26th, 2010 @ 14:26 ( . )

In single ended class A use with close to your volatges a 50L6 does require a 2k primary impeadance.
The red lead of the OT goes to the B+ node and the blue goes to pin 4.
If you read the posting in this section titled "odd ball output transfomer" I think I saw in there the procedure for testing a OTs impeadance, if not then do a search as its been covered on here alot.

-- REPLY: [With No Quote] --- [With Quoted Text]

'screen voltage too high SE 50L6 amp'
Author:Dust Puppy (registered user: 158 posts )
Date: Wed, Oct 27th, 2010 @ 15:51 ( . )

On 10/26/2010 @ 14:26, steve25 wrote :
In single ended class A use with close to your volatges a 50L6 does require a 2k primary impeadance.
--


If you need an OT, you can email me at engine94 at gmail dot com,
I have an Edcor GXSE-8-2.5 OT collecting dust.

-- REPLY: [With No Quote] --- [With Quoted Text]

'screen voltage too high SE 50L6 amp'
Author:Doug H (registered user: 5398 posts )
Date: Tue, Oct 26th, 2010 @ 14:35 ( . )

In this case transformer leads being swapped won't make a difference. You need a lower primary impedance xformer too (8k is too high) but that's not going to make a difference for the DC quiescent state. One thing I do see though is that the 180 ohm Rk looks pretty big. It must be biased very cold. I just built a 50L6 amp like this with similar voltages and at 150ohm Rk, it was still only dissipating 5W of 10W allowed on the plate. I ended up using a 90 ohm resistor there and it did two things:

1. It biased the plate a little hotter (7W- still well within spec).

2. It drew down the plate *and* screen voltage some. The screen voltage was on the edge of being out of spec, maybe a little over like yours. Now it is well within the safe limits.

I would try lowering that cathode resistor some. And remember when you measure the screen voltage you can subtract the cathode voltage from that when checking to see if it is in spec. The max voltage on the data sheet is the "voltage across the screen", which doesn't include the cathode to ground voltage.

To measure the OT primary windings, disconnect them from the circuit and put your DVM in resistance mode on each primary lead.

-- REPLY: [With No Quote] --- [With Quoted Text]

'screen voltage too high SE 50L6 amp'
Author:Mors08 (registered user: 92 posts )
Date: Wed, Oct 27th, 2010 @ 13:54 ( . )

On 10/26/2010 @ 14:35, Doug H wrote :
In this case transformer leads being swapped won't make a difference. You need a lower primary impedance xformer too (8k is too high) but that's not going to make a difference for the DC quiescent state. One thing I do see though is that the 180 ohm Rk looks pretty big. It must be biased very cold. I just built a 50L6 amp like this with similar voltages and at 150ohm Rk, it was still only dissipating 5W of 10W allowed on the plate. I ended up using a 90 ohm resistor there and it did two things:
:
: 1. It biased the plate a little hotter (7W- still well within spec).
:
: 2. It drew down the plate *and* screen voltage some. The screen voltage was on the edge of being out of spec, maybe a little over like yours. Now it is well within the safe limits.
:
: I would try lowering that cathode resistor some. And remember when you measure the screen voltage you can subtract the cathode voltage from that when checking to see if it is in spec. The max voltage on the data sheet is the "voltage across the screen", which doesn't include the cathode to ground voltage.
:
: To measure the OT primary windings, disconnect them from the circuit and put your DVM in resistance mode on each primary lead.
--


So I tried dumping down the cathode resistor - no luck with voltage changes. I put another resistor from B+ to pin4 at 1.5k - no change.

When I measured OT resistance it only read 400 ohms but this is only DC resistance not it's real impedance, yes?
Whatever it is, it's clearly not the right OT for this, no?

How about one of these?
[link]

Will 1.5k work with the 50L6 tube? Can one add a few larger watt resistors in line with this to up it's primary resistance?

-- REPLY: [With No Quote] --- [With Quoted Text]

'screen voltage too high SE 50L6 amp'
Author:PhilS (registered user: 1586 posts )
Date: Wed, Oct 27th, 2010 @ 15:28 ( . )

On 10/27/2010 @ 13:54, Mors08 wrote :
: How about one of these?
: [link]
:
: Will 1.5k work with the 50L6 tube? Can one add a few larger watt resistors in line with this to up it's primary resistance?

That is a push-pull OT. You should really get a SE OT. The important difference is in the construction. A SE amp has the whole OT "on" all the time, so it is constructed with an air gap to prevent over saturation and over heating. Simplistically, with PP OT, only one half is conducting in a single cycle, so each half turns on and off, and these are built without the air gap.

"Fixing" the primary resistance with a resistor doesn't work. Transformers are all about turns ratios and the resistor does nothing in that department.

Actually, resistance isn't the issue. Impedance is the thing we are discussing. In time, I think you will come to understand when these are different and why. I'm not the right one to give a succinct explanation.

-- REPLY: [With No Quote] --- [With Quoted Text]

'screen voltage too high SE 50L6 amp'
Author:Doug H (registered user: 5398 posts )
Date: Thu, Oct 28th, 2010 @ 08:49 ( . )

If you have the screen and plate tied to the same power supply there will little if any difference in the two voltages. IMO I wouldn't worry about it as it doesn't sound like you're running the 50L6 anywhere near max. dissipation anyway.

-- REPLY: [With No Quote] --- [With Quoted Text]

'screen voltage too high SE 50L6 amp'
Author:Doug H (registered user: 5398 posts )
Date: Thu, Oct 28th, 2010 @ 11:38 ( . )

On 10/28/2010 @ 08:49, Doug H wrote :
If you have the screen and plate tied to the same power supply
--



Sorry, I meant to type "power supply node".

-- REPLY: [With No Quote] --- [With Quoted Text]

'screen voltage too high SE 50L6 amp'
Author:Dust Puppy (registered user: 158 posts )
Date: Thu, Oct 28th, 2010 @ 14:05 ( . )

Is it possible you have B+ and B++ switched on the 50L6?

-- REPLY: [With No Quote] --- [With Quoted Text]


--- 24 Users Online --- 47 Recent Unique Posters

Q142=1411061436 - Threads: / 1411061436